A night's sleep consists of a series of sleep cycles each lasting 90-110 minutes in duration, with an average adult experiencing 4-5 of these cycles each night. A typical sleep cycle has five stages, which may be further categorized into non-rapid eye movement (NREM) and rapid eye movement (REM) sleep stages. NREM and REM sleep stages may be identified through physiological patterns.
There are four stages of NREM sleep. The lighter stages of NREM sleep appear first (stages 1 and 2), and often alternate with brief episodes of wakefulness before the deeper NREM stages are entered (stages 3 and 4). The REM sleep stage appears at around 90 minute intervals. As the night progresses the REM sleep stages become longer and NREM sleep stages become both shorter and lighter. Brief arousals to wakefulness are a normal feature of sleep.
The stages of sleep offer different benefits to the sleeper. REM sleep, or dream sleep, is essential to our minds for processing and consolidating emotions, memories and stress. It is also thought to be vital to learning, stimulating the brain regions used in learning and developing new skills. Most of dreaming occurs during REM sleep, although it can happen during other sleep stages as well. If REM sleep is disrupted one night, your body will go through more REM sleep the next night to catch up on this sleep stage. This may be an indication of the importance of REM sleep. However, deep sleep (stages 3 and 4 of NREM sleep) is perhaps the most vital stage. It is the first stage that the brain attempts to recover when sleep deprived, and the strongest effects of sleep deprivation are from inadequate deep sleep. Thus, getting adequate deep sleep (stages 3 and 4 of NREM sleep) and REM sleep should be encouraged.
A person is likely to have different experiences when awakening from each of the sleep stages. For example, the highest arousal thresholds (e.g., difficulty of awakening) are observed in NREM sleep stages 4 and 3 (deep sleep), respectively. Intrusion of wakefulness from stages 3 and 4 of NREM sleep is characteristic of “disorders of arousal”. Here, the transitions from stages 3 and 4 of NREM sleep to wakefulness are incomplete or gradual and features of both states coexist temporarily. The commonest and mildest example of this is sleep inertia, in which sleep is followed by drowsiness or at least a sensation of feeling un-refreshed and of being no more alert after sleep than beforehand. Sleep inertia is often accompanied with temporary disorientation, often associated with physical in-coordination. Sleep inertia often lasts 30-120 minutes after waking in the morning. Sleep inertia may be the most common after oversleeping, after daytime naps if stages 3 and 4 of NREM sleep are entered, which is usual if a nap lasts for more than 30 minutes. Intrusion of wakefulness after REM sleep may be responsible for sleep paralysis at the end of a period of sleep. Awareness of the environment is accompanied by the motor inhibition of REM sleep.
Older people frequently experience insomnia due to age-related changes in sleep. The sleep cycle in this population is characterized by lighter sleep, more frequent awakenings, and less total sleep time. In fact, the relative amount of REM sleep generally decreases with age. Research has shown that the negative effects of poor sleep quality include reduced cognitive ability, a greater propensity for falls, low energy levels, potential for dizziness, drowsiness, etc. In fact, falling is a major cause of injury and mortality in elderly people.